Why did scientists using classical Newtonian physics have difficulty explaining the photoelectric effect?

Answer 1

Classical electrodynamics said that light energy was a wave, and that atoms absorbed that energy the same way that an object absorbs radiant heat. Thus, the more intense the light, the more energy would be absorbed by the atoms in a metal. When the electrons in a metal got enough energy from the incoming wave, it would be possible for that electron to shoot out from the atoms in the metal. The more energy absorbed, the more energetic would be the electrons coming from the metal. Also, until each atom got enough light energy, no electrons could possibly be expelled. Light frequency didn't matter nearly as much as intensity.

The photo-electric effect defied this approach in many ways:

1) No matter how intense was the light, if it was below a certain frequency, no electrons came out. If the light was above this frequency, increasing the intensity increased the NUMBER of electrons coming out, but not their energy.

2) No matter how dim was the light, electrons were coming out of the metal almost instantaneously.

3) Increasing the frequency of the light resulted in more energetic electrons coming from the metal, even if the intensity was decreased.

Explaining this via classical electrodynamics was pretty much impossible. Einstein showed that a VERY radical assumption made explaining all of them almost trivial.